Ideally this type of interaction would be possible natively in the Designer, but I haven’t heard any rumblings of this being worked on. That said, the team tends to be tight-lipped when it comes to sharing specific details on the roadmap so it may be a part of a planned interactions updated.
Coincidentally there is a live stream with Vlad this Wednesday and the team is asking users to submit questions here that can be answered in the Q&A section. I can’t say whether he’ll have any concrete information but it may be something he can weigh in on.
As far as solutions available now, I would forgo using Lottie image sequences for anything over a handful of frames as file sizes tend to be extremely large. Realistically 15-20mb is too big and I’d imagine you’d need a fair bit of compression to even get it to that point. Your best bet is using custom code to scrub through video frames as mentioned in my previous link.